A device to device (D2D) discovery process is a process of determining whether a D2D-enabled user equipment (UE) is in proximity of other D2D-enabled UE. A discovering D2D-enabled UE determines whether another D2D-enabled UE is of interest to the discovering D2D-enabled UE based on the D2D discovery process. The other D2D-enabled UE is of interest to the discovering D2D-enabled UE if proximity of the other D2D-enabled UE needs to be known by one or more authorized applications on the discovering D2D-enabled UE. For example, a social networking application may be enabled to use a D2D discovery feature. The D2D discovery process enables a D2D-enabled UE of a given user of a social networking application to discover D2D-enabled UEs of friends of the given user of the social networking application, or to be discoverable by the D2D-enabled UEs of the friends of the given user of the social networking application. In another example, the D2D discovery process may enable the D2D-enabled UE of a given user of a search application to discover stores/restaurants, and the like, of interest of the D2D-enabled UE of the given user of the search application in proximity of the D2D-enabled UE of the given user of the search application.
A radio spectrum or radio frequency used for D2D communication is identical to a radio spectrum or radio frequency used for general communication between a UE and base station (BS). For example, in a frequency division duplex (FDD) system, a D2D transmission and a transmission from a UE to a BS may be configured on an uplink (UL) frequency. Radio resources for a D2D transmission are allocated or reserved on the UL frequency. In a time division duplex (TDD) system, radio resources, i.e., UL radio frames are reserved for a D2D transmission.
One issue of coexistence of a D2D transmission and a transmission from a UE to a BS in UL resources is collision between transmissions related to a random access and D2D transmissions.
For communication, a wireless link is divided into time slots, i.e., radio frames. Length of each radio frame may be 10 ms. Each radio frame is further divided into 10 sub-frames, and a length of each sub-frame may be lms. A sub-frame is a minimum unit for transmitting a data packet, and is called a transmit time interval (TTI).
A random access operation in a communication system supporting a D2D scheme according to the related art will be described with reference to FIG. 1.
FIG. 1 schematically illustrates a random access operation in a communication system supporting a D2D scheme according to the related art.
Referring to FIG. 1, during a random access, a UE selects a sub-frame based on physical random access channel (PRACH) resource configuration, and transmits a PRACH preamble, i.e., a message MSG 1 in a sub-frame x. Here, the PRACH resource configuration is broadcasted by a BS. After transmitting the PRACH preamble in the sub-frame x, the UE waits for a random access response (RAR) from the BS within a RAR window which starts after 3 sub-frames that follow the sub-frame x. For example, the RAR window may be set to an interval including up to 10 sub-frames. The RAR will be called a message MSG 2. The BS transmits the RAR in a sub-frame n within the RAR window. The UE receives the RAR in the sub-frame n within the RAR window, and transmits a message MSG 3 in a sub-frame n+6. Here, RAR window configuration for the RAR window size is broadcasted by the BS. For example, the RAR window configuration includes information on a size of the RAR window.
A random access operation in a communication system supporting a D2D scheme according to the related art has been described with reference to FIG. 1, and D2D resource configuration in a communication system supporting a D2D scheme according to the related art will be described with reference to FIG. 2.
FIG. 2 schematically illustrates D2D resource configuration in a communication system supporting a D2D scheme according to the related art.
Referring to FIG. 2, a resource for a PRACH, i.e., a PRACH resource may be included in all sub-frames included in every radio frame.
Alternatively, the PRACH resource may be included in a specific sub-frame which is included in specific radio frames, e.g., even numbered radio frames.
Alternatively, the PRACH resource may be included in a specific sub-frame which is included in every radio frame.
Meanwhile, resources for a D2D transmission, i.e., D2D sub-frames are periodically configured. For example, as illustrated in FIG. 2, 32 sub-frames are allocated for a D2D transmission every 1 second. Here, a resource for a D2D scheme will be called a D2D resource, a period by which the D2D resource is configured will be called a D2D resource cycle, and duration which is occupied by the D2D resource will be called D2D resource duration.
Meanwhile, periodic D2D resource configuration is associated with two main issues which are related to a PRACH collision.
The first issue is that a preamble transmission through a PRACH resource may collide with D2D transmissions.
The second issue is that an MSG 3 transmission in a sub-frame may collide with a D2D transmission.
That is, a D2D transmission will collide with a random access transmission. The random access transmission may include a preamble transmission through a PRACH resource or an MSG 3 transmission.
So, there is a need for avoiding a collision between a D2D transmission and a random access transmission in a communication system supporting a D2D scheme.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.